Efficient rolled thin-gauge L450M pipeline steel plate and manufacturing method thereof

Abstract

The invention relates to the technical field of low-carbon microalloyed steel production, in particular to a high-efficiency rolled thin L450M pipeline steel plate and a manufacturing method thereof, the steel comprises the following chemical components in percentage by weight: 0.05%-0.07% of C, 0.10%-0.25% of Si, 1.45%-1.55% of Mn, less than or equal to 0.020% of P, less than or equal to 0.015% of S, 0.05%-0.06% of V, 0.05%-0.10% of Cr, 0.008%-0.020% of Ti, 0.015%-0.04% of Al, 0.005%-0.009% of N and the balance of Fe and inevitable impurities, and the total amount of impurity elements is less than 0.05%; the production process of the steel plate comprises the steps of molten steel smelting, external refining and degassing, continuous casting, casting blank heating, controlled rolling, controlled cooling and air cooling to the room temperature. The method is used for manufacturing the high-strength L450M hot-rolled steel plate for the oil pipeline with the thickness specification being 9-15 mm and the pipe diameter being phi 914 mm or below, the problems that the steel plate rolling efficiency is low, the rolling resistance is large, the steel plate performance uniformity and the plate shape are not easy to control and the like are solved, and the steel plate has the advantages of being low in cost, high in strength and toughness, high in strength, excellent in low-temperature toughness and the like.

Description

Technical Field

[0001] This invention relates to the field of low-carbon microalloyed steel production technology, and in particular to a high-efficiency rolled thin-gauge L450M pipeline steel plate and its manufacturing method. Background Technology

[0002] In recent years, the construction of energy transmission pipelines has developed rapidly. To improve transmission efficiency and operational safety, the requirements for the strength and toughness of the selected steel raw materials are becoming increasingly stringent. Currently, major domestic and international pipeline trunk lines mainly use L485M and L555M steel grades, while branch lines and urban pipeline networks mostly use thinner (9-15mm) and smaller diameter (Φ914mm and below) L450M steel grades. When pipelines are located in extremely cold regions, from the perspective of operational stability and safety, higher requirements are placed on the low-temperature toughness indicators of raw materials. For example, the required testing temperatures for low-temperature drop hammer tests and ductile-brittle transition temperatures are lower, and the required Charpy impact energy is higher. Therefore, these projects have put forward higher personalized requirements, and simply meeting the API-SPEC-5L specification is far from sufficient. In addition, facing the severe market situation, user requirements, and manufacturing cost pressures in the steel industry, it is particularly important to reduce alloy costs, increase rolling speed, and minimize resource consumption and maximize manufacturing efficiency on the production line while ensuring product quality. However, steel plates with reduced alloy content are typically produced using a two- or even three-stage controlled rolling process. To ensure the cumulative reduction rate during the finishing rolling stage and to facilitate static recrystallization in the intermediate billet, a relatively thick intermediate billet is required for heating. This results in a prolonged heating time for the intermediate billet, and the final rolling temperature may be controlled below 820℃ or even lower, significantly increasing the rolling difficulty. Firstly, mill load, steel plate temperature uniformity, and plate shape control all face severe challenges. Secondly, improving the steel plate's performance depends primarily on the content of impurities such as phosphorus and sulfur, the content of alloying elements, and the control of key process parameters in smelting and rolling. Therefore, solving the aforementioned problems in producing thin-gauge L450M pipeline steel, while simultaneously improving rolling efficiency and ensuring its excellent strength and toughness, is crucial for the development of thin-gauge L450M pipeline steel.

[0003] Compared with existing technologies:

[0004] To date, there are very few reports, both domestically and internationally, on improving the rolling efficiency of thin-gauge L450M pipeline steel. Prior to this invention, patent application number CN201010243241.0 disclosed an X65 pipeline steel and its production method. The weight percentage composition of this patent is C: 0.055%~0.090%, Si: 0.15%~0.35%, Mn: 1.50%~1.65%, P≤0.020%, S≤0.005%, Nb: 0.040%~0.055%, V: 0.040%~0.070%, Ti: 0.010%~0.025%, N≤0.008%, Als: 0.005%~0.060%. However, this steel grade uses natural air cooling, which is inefficient and cannot fully utilize the water toughening effect. In addition, the specific strength and toughness of the actual product are not clearly defined.

[0005] Application number CN201110179945.0 discloses an X65 pipeline steel with excellent low-temperature toughness and its manufacturing method. The weight percentage of the components in this patent is C: 0.020%~0.055%, Si: 0.10%~0.25%, Mn: 1.50%~1.70%, Nb: 0.060%~0.080%, Cr: 0.20%~0.35%, V: 0.020%~0.040%, Ti: 0.010%~0.020%, Als: 0.010%~0.040%, P≤0.018%, S≤0.005%, N≤0.006%. However, this steel contains Nb and Cr, which increases the cost of alloying. The laminar flow cooling process cannot fully utilize the water-substitution alloy to reduce costs and improve strength and toughness.

[0006] Application No. KR20020027013(A) discloses an API-X65 pipeline pipe with good aging performance and its manufacturing method. The weight percentage of the components in this patent is C: 0.07%~0.09%, Si: 0.2%~0.5%, Mn: 1.40%~1.60%, P≤0.025%, S≤0.005%, Nb: 0.035%~0.045%, V: 0.04%~0.05%, Mo: 0.03%~0.07%, Cr: 0.05%~0.15%, Ti: 0.005%~0.015%, N: 0.002%~0.007%, Als: 0.015%~0.050%. However, this steel contains Mo, which increases the cost and results in insufficient low-temperature toughness of the product.

[0007] Although the steels disclosed in the above patent documents have achieved high strength and toughness, they are either coils or have high production costs and low rolling efficiency, and are therefore not suitable for producing low-cost, thin-gauge, high-straightness L450M pipeline steel plates, etc. Summary of the Invention

[0008] This invention provides a method for manufacturing a high-efficiency rolled thin-gauge L450M pipeline steel plate and a method for manufacturing the same. It is used to manufacture high-strength L450M hot-rolled steel plates for oil pipelines with a thickness of 9-15mm and a diameter of Φ914mm and below. This invention solves problems such as low rolling efficiency, high rolling resistance, uniform steel plate performance, and difficulty in controlling plate shape. The steel plate has low cost, high strength and toughness, and features high strength and excellent low-temperature toughness, which can ensure the safety of oil pipelines.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A high-efficiency rolled thin-gauge L450M pipeline steel plate, wherein the chemical composition of the steel by weight percentage is: C: 0.05%~0.07%, Si: 0.10%~0.25%, Mn: 1.45%~1.55%, P≤0.020%, S≤0.015%, V: 0.05%~0.06%, Cr: 0.05%~0.10%, Ti: 0.008%~0.020%, Al: 0.015%~0.04%, N: 0.005%~0.009%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0011] Furthermore, the finished thickness of the L450M pipeline steel plate is 9-15mm, and it is used to manufacture oil pipelines with a diameter of Φ914mm and below.

[0012] Furthermore, the mechanical properties of the finished steel plate are as follows: the yield strength in the transverse tensile test is between 480 and 520 MPa, the tensile strength is between 570 and 630 MPa, the yield-to-tensile ratio is less than 0.9, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥260 J, and the transverse shear area SA at -20℃ DWTT is not less than 95%.

[0013] A method for manufacturing high-efficiency rolled thin-gauge L450M pipeline steel plates, the steel plate production process includes steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, wherein...

[0014] 1) Steel smelting: Smelting is carried out according to composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, the raw materials are fed into the converter. In the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.05-0.07%. Argon gas is blown for 10-15 minutes when tapping the steel.

[0015] 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 15 minutes;

[0016] 3) Continuous casting: Slabs are continuously cast with a superheat of 9-14℃ and a casting speed of 0.7-1.0m / min. In the horizontal sector section, i.e. the end of solidification, heavy pressure is applied to the continuously cast slab, and the reduction is 10-20mm. After the slabs are removed from the line, they are stacked and cooled slowly with a stacking temperature of not less than 800℃ and a cooling time of not less than 36h.

[0017] 4) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 900-1150℃, the temperature range of the heating section is 1200-1210℃, and the temperature range of the soaking section is 1100-1120℃. The time in the heating and soaking sections is controlled to be 3-4 hours, of which the soaking section time is not less than 0.5 hours.

[0018] 5) Controlled rolling: Before rolling, use high-pressure water to descale the billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; rolling is divided into two stages: the first stage is recrystallization rolling, i.e. rough rolling, with an initial rolling temperature range of 1100-1120℃ and a final rough rolling temperature range of 990-1030℃. The rough rolling should not exceed six passes, and the reduction ratio of the first four passes should be guaranteed to be above 20% for two passes. The thickness of the intermediate billet should be 1.5-2.0 times that of the finished product. The last two passes of the rough rolling stage are rolled with descaling water sprayed into the rolling mill for each pass, with a descaling time of 0.5-1 minute and a descaling machine pressure of 15-25 MPa.

[0019] The second stage is non-recrystallization rolling, i.e., finishing rolling. After roughing, the rolling process proceeds directly to the finishing stage without waiting for the temperature to reach the desired level. The initial rolling temperature range is 980–1020℃, and the final rolling temperature range is 850–890℃. There are no more than four finishing rolling passes. The reduction ratio for the first two passes is guaranteed to be above 20%. The final pass uses a reduction ratio of less than 3%, with increased speed rolling and rapid steel removal after rolling. The steel removal speed is 4–6.5 m / s, and pre-straightening is implemented.

[0020] 6) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 760–770℃ and a final cooling temperature range of 560–590℃. The cooling rate is controlled at 8–15℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water flow rate are 2–3MPa and 35–50m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.6mm, the outlet roller position is -2.9mm~-3.7mm, and then it is air cooled to room temperature.

[0021] Furthermore, the thickness of the continuously cast billet is 150-200 mm, and it is rolled on a medium-thick plate reciprocating rolling mill.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] 1) After deep desulfurization pretreatment of KR hot metal, the slag is thoroughly removed. The converter uses a double-slag method for P removal, resulting in lower P and S content in the billet. By controlling the argon blowing time and maintaining the RH vacuum degassing time, defects such as central segregation, inclusions, and excessive H and O content in the billet caused by high Mn, Cr, and C content are overcome. This is beneficial for improving the plasticity and toughness of pipeline steel. Reducing superheat and continuous casting speed and placing the billet under heavy pressure can improve macroscopic segregation in the continuous casting billet, reduce the spacing of secondary dendrite arms in the solidification structure of the continuous casting billet, and help reduce billet segregation and internal structural defects. Placing the billet under heavy pressure at the end of solidification helps refine the austenite grains, and after rolling, it can reduce the width of ferrite bands in the banded structure. At the same time, the billet is stacked and slowly cooled after leaving the production line, and the stacking temperature and slow cooling time are limited, which helps promote the diffusion of Mn, Cr, and H elements and reduce their impact on the microstructure and properties due to compositional segregation.

[0024] 2) The billet heating process involves limiting the temperature and time of the preheating, heating, and soaking stages to ensure that Cr and Ti carbides and nitrides dissolve rapidly and fully in the matrix and diffuse sufficiently. This also promotes the diffusion of alloying elements Mn, Cr, and C, mitigating their impact on microstructure and properties due to compositional segregation. Furthermore, it suppresses the impact of primary austenite grain coarsening on DWTT performance. Controlling the temperature and time of the heating and soaking stages (low-temperature heating) further inhibits excessive austenite grain growth, increasing the contribution of fine-grain strengthening to the steel plate's strength. Additionally, the low-temperature soaking stage reduces the waiting time for the second-stage rolling process, improving rolling efficiency.

[0025] 3) The composition of this invention is reasonable and the amount of alloy added is low. By replacing Nb and Mo with V and adding a small amount of micro-alloying element Cr, the cost of alloy and the resistance to high-temperature deformation in the roughing and finishing stages are greatly reduced, which is conducive to increasing the reduction of each pass and ensuring the comprehensive performance of the super steel plate.

[0026] 4) A two-stage controlled rolling process is adopted, controlling the reduction rate of the roughing and finishing passes. Utilizing the high-temperature deformation-induced effect, the precipitation of the VN phase within the austenite grains is promoted, providing nucleation sites for acicular ferrite, thereby promoting the formation of intragranular acicular ferrite and improving strength and toughness. The intermediate billet thickness is optimized, while the finishing rolling start temperature is increased (no waiting time is required during the finishing stage). A flexible high-pressure water descaling process is used to inhibit grain growth, ensure core rolling penetration, and suppress the formation of wide banded structures in the core. Simultaneously, the intermediate billet waiting time is shortened, and no relaxation waiting is required after rolling, improving rolling efficiency. A small reduction rate is used in the final pass to flatten the steel plate shape, reduce internal stress, and control the length of the rolled steel plate to avoid excessively rapid temperature drop in later rolling stages, which could affect the uniformity of the steel plate structure after controlled cooling. Rapid post-rolling... The process involves steel polishing, pre-straightening, and adjusting the cooling rate to control the initial cooling temperature range of the steel plate at 760–770℃ and the final cooling temperature at 560–590℃. This ensures that the proportion of acicular ferrite is 30% or higher (through calculation and experiment, the fastest precipitation temperature range for VN is 760–780℃; controlling the initial cooling temperature ensures the density of VN nano-precipitations, which can significantly improve the mechanical properties of the steel through precipitation strengthening and promote the transformation of acicular ferrite. Based on this, a high reheat temperature and slow cooling rate are adopted to avoid excessive internal stress in the steel plate due to excessively fast cooling, resulting in poor plate shape). This ensures the low-temperature DWTT performance of the steel plate. Side spraying, air blowing, and hot straightening are used to facilitate the control of the steel plate shape, improve the uniformity of steel plate performance, reduce the probability of problems with the beginning and end of the plate shape, and save on the investment cost of subsequent cold straightening equipment.

[0027] 5) This invention reduces alloy costs through simple composition design and obtains an economical L450M pipeline steel plate for oil pipelines with a thickness specification of 9-15mm and a pipe diameter of Φ914mm and below by controlling the steelmaking, continuous casting, heating, controlled rolling and cooling, and hot straightening processes. The microstructure is a multiphase structure mainly composed of acicular ferrite, pearlite, and polygonal ferrite, with acicular ferrite accounting for more than 30%. The rolling efficiency is increased by 10-35s per piece, and the steel plate has good low-temperature toughness. The specific properties are: the yield strength in the transverse tensile test is between 480-520MPa, the tensile strength is between 570-630MPa, the strength range is narrow, the yield strength ratio is less than 0.9, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥260J, and the transverse shear area SA at -20℃ is not less than 95%. Detailed Implementation

[0028] The specific embodiments of the present invention will be further described below:

[0029] This invention discloses a high-efficiency rolled thin-gauge L450M pipeline steel plate and its manufacturing method. The finished L450M hot-rolled steel plate for oil transportation has a thickness of 9-15mm. It is produced using continuously cast billets with a thickness of 150-200mm on a medium-thick plate reciprocating mill, with water as the cooling medium. It is used to manufacture high-strength L450M hot-rolled steel plates for oil pipelines with a diameter of Φ914mm and below. The chemical composition by weight percentage is: C: 0.05%-0.07%, S... i: 0.10%~0.25%, Mn: 1.45%~1.55%, P≤0.020%, S≤0.015%, V: 0.05%~0.06%, Cr: 0.05%~0.10%, Ti: 0.008%~0.020%, Al: 0.015%~0.04%, N: 0.005%~0.009%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%.

[0030] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0031] C: The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness and weldability of steel. Therefore, the present invention sets the C content range to 0.05% to 0.07%.

[0032] Mn: It can improve the strength of steel through solid solution strengthening and compensate for the strength loss of steel plate caused by the reduction of C content. In addition, it can lower the γ-α phase transformation temperature, thereby refining the ferrite grains and helping to obtain fine low-temperature phase transformation products, thus improving its toughness. However, increasing the Mn content will aggravate the center segregation of the continuously cast billet, which is not conducive to improving the low-temperature toughness of the steel plate and cannot guarantee the uniformity of the cross-sectional structure of the steel plate. Therefore, the Mn content range of this invention is designed to be 1.45% to 1.55%.

[0033] Si plays a role in deoxidation in steelmaking and improving the strength of the matrix. However, excessive Si will reduce the toughness of the heat-affected zone of the base material. Increasing the Si content can purify ferrite and reduce the content of pearlite, which is beneficial to reducing the Bauschinger effect of the matrix material. Therefore, the Si content is set to 0.10% to 0.25% in this invention.

[0034] V: is a strong solid nitrogen element that exists in the form of VN in continuously cast billets. Fine VN particles can effectively inhibit the growth of austenite grains during the reheating of continuously cast billets. During the rolling stage, it can precipitate in the high-temperature austenite region, providing phase deformation nuclei for acicular ferrite; and form nano-precipitates during slow cooling, which can significantly improve the strength of steel and improve the impact toughness of the weld heat-affected zone. When the amount of V added exceeds a certain value, the V particles will coarsen, increasing the stress concentration level between the particle interface and the matrix. Therefore, the present invention selects a V content range of 0.05% to 0.06%.

[0035] N: In steel, nitrogen (N) has no other significant role besides forming fine TiN particles to refine austenite grains. Therefore, it needs to be kept at a low content level. The N content range selected in this invention is 0.005% to 0.009%.

[0036] Ti is a strong solid nitrogen element, existing in the form of TiN in continuously cast billets. Fine TiN particles can effectively inhibit the growth of austenite grains during the reheating of continuously cast billets and help improve the solid solubility of V in austenite, thereby improving the impact toughness of the weld heat-affected zone. When the amount of Ti added exceeds a certain value, the TiN particles will coarsen, increasing the stress concentration level between the particle interface and the matrix. Therefore, the present invention selects a Ti content range of 0.008% to 0.02%.

[0037] Al: It is usually used as a deoxidizer in steel. If AlN is formed, it can also refine the microstructure. When the Al content exceeds 0.04%, the excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, the deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit of Al content is set at 0.015%.

[0038] Cr: It is a major element that can effectively improve hardenability, inhibit ferrite formation and promote bainite formation. It plays an important role in controlling phase transformation structure, promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, and improves the strength, plasticity and toughness of steel plate. The Cr content range selected in this invention is 0.05% to 0.10%.

[0039] P and S are unavoidable impurity elements in steel, and the lower the better. However, due to considerations of smelting costs and processes, they cannot be kept indefinitely low. Therefore, this invention sets the upper limit of P and S content to 0.020% and 0.015%, respectively.

[0040] This invention discloses a method for manufacturing high-efficiency rolled thin-gauge L450M pipeline steel plates. The production process includes steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature.

[0041] 1) Steel smelting to continuous casting: The steel is smelted according to the following composition by weight percentage: C: 0.05%–0.07%, Si: 0.10%–0.25%, Mn: 1.45%–1.55%, P≤0.020%, S≤0.015%, V: 0.05%–0.06%, Cr: 0.05%–0.10%, Ti: 0.008%–0.020%, Al: 0.015%–0.04%, N: 0.005%–0.009%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%; the raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%, and after slag removal, they enter the converter; the converter... During smelting, a double-slag method is used to remove phosphorus (P), controlling the P content to ≤0.02%. At the end of converter smelting, the C content is controlled to be 0.05-0.07%. Argon gas is blown for 10-15 minutes during tapping. Then, LF refining and RH vacuum degassing are carried out, with RH vacuum maintained for more than 15 minutes. After that, slab continuous casting is carried out, with a superheat of 9-14℃ and a casting speed of 0.7-1.0 m / min. In the horizontal sector section, i.e., at the end of solidification, heavy pressure is applied, and the continuous casting billet is reduced by 10-20 mm. After the billet is removed from the line, it is stacked and slowly cooled, with a stacking temperature of not less than 800℃ and a slow cooling time of not less than 36 hours, to promote the diffusion of Mn, Cr, and H elements and reduce their influence on microstructure and properties due to component segregation.

[0042] 2) Billet Heating: A billet with a thickness of 150-200mm is fed into a walking beam furnace for heating. The billet passes through a preheating section, a heating section, and a soaking section before exiting the furnace. The temperature range of the preheating section is 900-1150℃, which promotes the rapid and complete dissolution of Cr and Ti carbides and nitrides into the matrix and allows for sufficient diffusion. The temperature range of the heating section is 1200-1210℃, and the temperature range of the soaking section is 1100-1120℃. The time spent in the heating and soaking sections is controlled at 3-4 hours, with the soaking section lasting no less than 0.5 hours. This further promotes the diffusion of alloying elements Mn, Cr, and C, reducing their impact on microstructure and properties due to compositional segregation. At the same time, it also suppresses the impact of coarsening of the original austenite grains on DWTT performance.

[0043] 3) Control rolling to air cooling to room temperature: Before rolling, use high-pressure water to descale the billet after it comes out of the furnace for 1 to 2 minutes, with a descaling machine pressure of 15 to 25 MPa;

[0044] The rolling process is divided into two stages: the first stage is recrystallization rolling, i.e., rough rolling, with an initial rolling temperature range of 1100–1120℃ and a final rough rolling temperature range of 990–1030℃. The rough rolling process consists of no more than six passes, with two of the first four passes ensuring a reduction rate of over 20%. The resulting intermediate billet thickness is 1.5–2.0 times the finished product thickness, reducing the intermediate billet's waiting-to-heat thickness, accelerating its temperature drop, and improving rolling efficiency (finishing does not require waiting-to-heat). After the first two rolling passes, descaling water is sprayed onto the mill for 0.5–1 minute at a pressure of 15–25 MPa to inhibit austenite grain growth and improve DWTT performance. The second stage is non-recrystallization rolling, i.e., finish rolling. After rough rolling, the mill directly enters the finish rolling stage without waiting for warm-up. The initial rolling temperature range is 980–1020℃, and the final rolling temperature range is 850–890℃. There are no more than four finish rolling passes, and the reduction ratio is guaranteed in the first two passes. To ensure core penetration and suppress the formation of wide banded structures, rolling is performed at a rate of over 20%. Two-stage rolling refines the grain structure at different stages and introduces a degree of pre-deformation to reduce grain size after phase transformation, improving the low-temperature toughness of the steel plate. The final pass uses a reduction rate of less than 3%. Rapid post-rolling steel removal is performed at a speed of 4–6.5 m / s. Pre-straightening is implemented to ensure the initial plate shape upon entering the water, while controlling the immersion temperature to suppress proprecipitated ferrite. Laminar flow cooling is then employed, with an initial cooling temperature range of 760–770℃ and a final cooling temperature range of 560–590℃, controlling the cooling rate at 8–15℃ / s. Slow cooling is used to ensure that the acicular ferrite proportion is 30% or higher, while reducing pearlite formation and suppressing wide banded structures, ensuring the low-temperature DWTT performance of the steel plate. After the steel plate exits controlled cooling, side spraying and air purging are activated, with side spray pressure and water volume of 2–3 MPa and 35–50 m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.6mm, the outlet roller position is -2.9mm~-3.7mm, and then it is air cooled to room temperature.

[0045] By adopting the above-mentioned composition and steelmaking continuous casting, controlled rolling and controlled cooling scheme, the shortcomings of the existing technology have been overcome, and an economical L450M pipeline steel plate for high-efficiency rolling of thin-gauge (9-15mm) oil pipelines with a pipe diameter of Φ914mm and below has been realized. The microstructure of the steel plate is a multiphase microstructure mainly composed of acicular ferrite, pearlite and polygonal ferrite, of which acicular ferrite accounts for more than 30%.

[0046] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0047] Example:

[0048] Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the smelting and stacking process of the steel in the examples; Table 3 shows the billet heating process and high-pressure water descaling process before rolling of the continuously cast billet in the examples; Table 4 shows the rolling parameters of the steel in the examples; Table 5 shows the main process parameters of controlled cooling of the steel in the examples; and Table 6 shows the microstructure, properties and efficiency indicators of the steel plates in the examples.

[0049] Table 1: Chemical composition (wt, %) of embodiments of the present invention

[0050]

[0051] Note: Impurity elements in steel: P≤0.02%; S≤0.015%; O≤0.0050%; total amount of other impurity elements is less than 0.05%.

[0052] Table 2: Smelting and Stacking Process Regulations for Steel in Examples

[0053]

[0054] Table 3: Heating regime of billet and high-pressure water descaling process before rolling of continuous casting billet for the example steel

[0055]

[0056] Table 4: Rolling parameters of the steel in the examples

[0057]

[0058] Table 5: Main process parameters for controlled cooling of steel in embodiments of the present invention

[0059]

[0060] Table 6: Steel plate microstructure, properties and efficiency indicators of embodiments of the present invention

[0061]

[0062] Therefore, compared with the existing technology, the composition design and steelmaking continuous casting, heating and controlled rolling and cooling scheme of this invention overcomes the shortcomings of the existing technology and realizes the production and application of an economical L450M pipeline steel plate for high-efficiency rolling of thin-gauge (9-15mm) oil pipelines with a pipe diameter of Φ914mm and below. The microstructure of the steel plate is a multiphase microstructure mainly composed of acicular ferrite, pearlite and polygonal ferrite, of which acicular ferrite accounts for more than 30%, and the steel plate has good low-temperature toughness.

[0063] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high-efficiency rolled thin-gauge L450M pipeline steel plate, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.05%~0.07%, Si: 0.10%~0.25%, Mn: 1.45%~1.55%, P≤0.020%, S≤0.015%, V: 0.05%~0.06%, Cr: 0.05%~0.10%, Ti: 0.008%~0.020%, Al: 0.015%~0.04%, N: 0.005%~0.009%, with the balance being Fe and unavoidable impurities, and the total amount of impurity elements being less than 0.05%. The method for manufacturing the high-efficiency rolled thin-gauge L450M pipeline steel plate, wherein: Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; rolling is divided into two stages: the first stage is recrystallization rolling, i.e., rough rolling, with an initial rolling temperature range of 1100-1120℃ and a final rough rolling temperature range of 990-1030℃. The rough rolling is limited to no more than six passes, and the reduction ratio for each pass is guaranteed to be above 20% for two of the first four passes. The thickness of the intermediate billet is 1.5-2.0 times that of the finished product. The last two passes of the rough rolling stage are rolled with descaling water sprayed into the rolling mill for each pass, with a descaling time of 0.5-1 minute and a descaling machine pressure of 15-25 MPa. The second stage is non-recrystallization rolling, i.e., finishing rolling. After roughing, the rolling process proceeds directly to the finishing stage without waiting for the temperature to reach the desired level. The initial rolling temperature range is 980–1020℃, and the final rolling temperature range is 850–890℃. There are no more than four finishing rolling passes. The reduction ratio for the first two passes is guaranteed to be above 20%. The final pass uses a reduction ratio of less than 3%, with increased speed rolling and rapid steel removal after rolling. The steel removal speed is 4–6.5 m / s, and pre-straightening is implemented. Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 760–770℃ and a final cooling temperature range of 560–590℃. The cooling rate is controlled at 8–15℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water flow rate are 2–3 MPa and 35–50 m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.6mm, the outlet roller position is -2.9mm~-3.7mm, and then it is air cooled to room temperature.

2. The high-efficiency rolled thin-gauge L450M pipeline steel plate according to claim 1, characterized in that, The finished thickness of the L450M pipeline steel plate is 9-15mm, and it is used to manufacture oil pipelines with a diameter of Φ914mm and below.

3. The high-efficiency rolled thin-gauge L450M pipeline steel plate according to claim 1, characterized in that, The mechanical properties of the finished steel plate are as follows: the yield strength in the transverse tensile test is between 480 and 520 MPa, the tensile strength is between 570 and 630 MPa, the yield-to-tensile ratio is less than 0.9, the elongation is ≥25%, the transverse Charpy impact energy at -35℃ is ≥260 J, and the transverse shear area SA at -20℃ is not less than 95%.

4. A method for manufacturing a high-efficiency rolled thin-gauge L450M pipeline steel plate as described in any one of claims 1 to 3, characterized in that, The steel plate production process includes steel smelting → ladle refining and degassing → continuous casting → billet heating → controlled rolling → controlled cooling → air cooling to room temperature, among which, 1) Steel smelting: Smelting is carried out according to composition. The raw materials are pretreated with KR hot metal to control the S content to be less than 0.015%. After slag removal, the raw materials are fed into the converter. In the converter smelting, the double slag method is used to remove P to control the P content to ≤0.02%. At the end of the converter smelting, the C content is controlled to be 0.05-0.07%. Argon gas is blown for 10-15 minutes when tapping the steel. 2) Ladle refining and degassing: Perform LF refining and RH vacuum degassing, maintaining RH vacuum for more than 15 minutes; 3) Continuous casting: Slabs are continuously cast with a superheat of 9-14℃ and a casting speed of 0.7-1.0m / min. In the horizontal sector section, i.e. the end of solidification, heavy pressure is applied to the continuously cast slab, and the reduction is 10-20mm. After the slabs are removed from the line, they are stacked and cooled slowly with a stacking temperature of not less than 800℃ and a cooling time of not less than 36h. 4) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 900-1150℃, the temperature range of the heating section is 1200-1210℃, and the temperature range of the soaking section is 1100-1120℃. The time in the heating and soaking sections is controlled to be 3-4 hours, of which the soaking section time is not less than 0.5 hours. 5) Controlled rolling: Before rolling, use high-pressure water to descale the billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; rolling is divided into two stages: the first stage is recrystallization rolling, i.e. rough rolling, with an initial rolling temperature range of 1100-1120℃ and a final rough rolling temperature range of 990-1030℃. The rough rolling should not exceed six passes, and the reduction ratio of the first four passes should be guaranteed to be above 20% for two passes. The thickness of the intermediate billet should be 1.5-2.0 times that of the finished product. The last two passes of the rough rolling stage are rolled with descaling water sprayed into the rolling mill for each pass, with a descaling time of 0.5-1 minute and a descaling machine pressure of 15-25 MPa. The second stage is non-recrystallization rolling, i.e., finishing rolling. After roughing, the rolling process proceeds directly to the finishing stage without waiting for the temperature to reach the desired level. The initial rolling temperature range is 980–1020℃, and the final rolling temperature range is 850–890℃. There are no more than four finishing rolling passes. The reduction ratio for the first two passes is guaranteed to be above 20%. The final pass uses a reduction ratio of less than 3%, with increased speed rolling and rapid steel removal after rolling. The steel removal speed is 4–6.5 m / s, and pre-straightening is implemented. 6) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 760–770℃ and a final cooling temperature range of 560–590℃. The cooling rate is controlled at 8–15℃ / s. After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water flow rate are 2–3MPa and 35–50m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, the hot straightening is followed by three straightening processes, the inlet roller position is -1.5mm~-2.6mm, the outlet roller position is -2.9mm~-3.7mm, and then it is air cooled to room temperature.

5. The method for manufacturing a high-efficiency rolled thin-gauge L450M pipeline steel plate according to claim 4, characterized in that, The thickness of the continuously cast billet is 150-200 mm, and it is rolled on a medium-thick plate reciprocating rolling mill.

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